The types of gene structural changes causing deficiency of hypoxanthine guanine phosphoribosyl transferase (HGPRT) activity in spontaneous mutations is being examined in cultured human fibroblasts. The deficiency of this enzyme activity causes a human disease (Lesch-Nyhan Syndrome). The restriction enzyme cleavage patterns of HGPRT gene sequences in mutant lines is being analyzed. The work is presently focused on obtaining a large number of independent spontaneous mutants that existed in newborn baby's foreskins. Forty independent mutants have been isolated from different normal newborns. The mutant cells were grown to large numbers. Portions of cultured cells were frozen in liquid nitrogen for cytogenetic and enzymology studies at a future time and portions of cultured cells were frozen for DNA extraction. Southern blot analysis is now in progress to assess the possible involvement of DNA rearrangements in spontaneous mutation.